email clients

Well, if you only use one device for your e-mail, you can set up POP for your e-mail. POP deletes downloads all your e-mail locally and then deletes it from the server.

I can tell you about Evolution, by default in Trisquel. Keeping the IMAP protocol (so that you can access your most recent e-mails from any computer), you only need to:

1. create a new folder "On This Computer" (or two: for sent and received mails), e.g., through the contextual menu you display with a right click;
2. select the emails to move (you probably want to order them by date, to click on the first email to move and to Shift+click on the last);
3. move them, using either the contextual menu or, easier, drag and drop.

That is all! Well, except that it may take hours to download gigabytes of emails. But you can configure folders (the "inbox" and/or the "sent box", etc.) so that every email they contain is, from now on, locally copied. It is only a box to check in the properties of the folder, which shows the percentage of the quota that you use too. Not only you will not have to download gigabytes of emails at once ever again but you can now read all your emails offline (well, the emails you received before going offline, of course): https://help.gnome.org/users/evolution/stable/mail-working-offline.html.en

For an easy search, Evolution can index the the bodies of the e-mails in folders "On This Computer". Again, it is only a box to check in the properties of the folder.

If you want to move from/to another another email client, you should search for import/export features. Here is Evolution's: https://help.gnome.org/users/evolution/stable/index.html.en#data-migration-and-sync (specific documentation to import from Thunderbird and Outlook; https://help.gnome.org/users/evolution/stable/import-single-files.html.en for a more generic solution that should work with Slypheed, assuming it can export to the MBOX format).

Evolution's data files are in ~/.local/share/evolution. Hidden sub-folders of ~/.local/share/evolution/mail/local (that reuse the name you chose for the folders) contain all the emails you downloaded, as separate text files. But directly dealing with them is not particularly user friendly: the name of the file contains when the email was downloaded (not sent!) as a number of seconds since 1970-01-01, the file contains the email headers, etc.

Evolution supports encryption with GPG, and so does Slypheed. If you want to test with me, you know my address and here is my public key: http://dcc.ufmg.br/~lcerf/Lo%C3%AFc%20Cerf.asc